The late-summer heat wave that has engulfed Maryland this week has caught the region's utility managers with their plants down.

Routine plant shutdowns, timed for the slack periods between the demand peaks of summer and winter, have left the power grid with barely enough reserve generating power to cope with record-breaking heat and humidity yesterday, officials said.

"We've just got everything loaded to the hilt," said Charles B. Woodward, operations manager at the Pennsylvania, New Jersey, Maryland Interconnection, which manages the electrical power grid serving 21 million people from New York to Washington.

For the first time this year, after the hottest summer ever recorded in downtown Baltimore, the Baltimore Gas and Electric Co. yesterday issued an appeal to its 1 million customers to conserve electricity.

"We'll probably have the same situation [today]," said BG&E spokesman Art Slusark.

The National Weather Service said record temperatures are "likely again today," with highs in the mid- to upper 90s and high humidity. It was already 86 degrees downtown at mid-morning, after an overnight low of 81 degrees at 6 a.m.

Baltimore County schools, which toughed out yesterday's heat, threw in the towel today and decided to dismiss students one hour early. City schools were to close at 12:30 p.m. for a second day.

The revised forecast for tomorrow calls for some relief, with temperatures in the upper 80s and continued high humidity. Thunderstorms tomorrow afternoon and evening will precede the passage of a cool front early Thursday, said forecaster Bob Melrose.

The new Canadian air behind the front is expected to bring slightly milder weather Thursday with highs near 80. If you can survive until Friday, highs then are expected only around 70, with overnight lows in the mid-40s in the colder suburbs, to the mid-50s the city.

But, for now, the September mini-heat wave is in its full glory, leaving BG&E struggling to deal with the power drain it causes.

"The forecasted [demand for electricity] was nowhere near what we got," Woodward said yesterday. "I've never seen the load pick up from what we call the 'midnight valley' to the load we have now."

Normally the demand for power across the regional power grid would climb 40 to 60 percent over the course of the day, he said.

Yesterday, "it probably doubled. That's a very healthy amount of electricity to come up with."

Slusark noted that, unlike peak demand periods in mid-summer, this time all area schools were in session and fewer people were on vacation, adding to demand.

Temperatures in Baltimore reached a record 98 degrees at Baltimore-Washington International Airport at 3:13 p.m., breaking the 97-degree record set in 1970. The high downtown reached 95 degrees at 2:25 p.m., tying a 1970 record for the date there.

New record highs were also set in Philadelphia, where it hit 94 degrees; at Washington's National Airport, where it reached 96 degrees at 3:51 p.m.; and at Dulles Airport in Virginia, where it was 97 degrees between 3 p.m. and 4 p.m. yesterday.

The demand for power from Baltimore Gas and Electric Co. customers reached 5,135 megawatts at 5 p.m. yesterday. That broke the previous September record of 4,930 megawatts, set in 1989, said Slusark.

Slusark said the utility easily met the record demand in July, partly because it had all its biggest generators in operation, including both 642-megawatt coal-fired generators at Brandon Shores in Anne Arundel County, and both nuclear generators at Calvert Cliffs.

But last Friday, Brandon Shores I, the older of the two units there, was shut down for four months so that its control panels can be replaced with updated equipment.

"It's a regional problem right now," Slusark said. "It's sort of the beginning of the time when plants start to go off-line for planned outages and upgrades."

Several generating stations in Pennsylvania, New Jersey and New York are also shut down now for routine maintenance, including the Peach Bottom 3 nuclear station in Delta, Pa., which is being refueled, Slusark said. Several unscheduled shutdowns complicated the picture.

To help reduce the demand for power, BG&E yesterday issued an appeal to residential customers -- the first this summer -- to conserve electricity by setting thermostats "as high as personal comfort permits. If you are going to be out of the house for more than four hours, turn the air conditioning off," the appeal said.